The test for a threat
A box qualifies as a threat if it passes one question: if nothing else happened, only this, would the top event occur? If the answer is "no, you would also need…", what you have is not a threat but a link in a chain.
Why the distinction matters
Because barriers sit on threats. Get the threat wrong and the barrier goes in the wrong place: write "lack of training" as a threat and every answer becomes training, while a hardware barrier is never considered.
Where deeper causes go
- Fatigue, shift patterns, understaffing → escalation factors on barriers that depend on people.
- Maintenance budget, spare part supply → escalation factors on hardware barriers.
- Production pressure → usually an escalation factor shared by several barriers, which makes it the most dangerous of all.
One cause, many barriers
If a single deeper cause weakens several barriers at once, those barriers are no longer independent. The bowtie shows five barriers while in reality one failure takes all five down. That exposure of false depth is the real prize of the distinction.
Relation to root cause analysis
Root cause analysis looks backwards after an event; a bowtie looks forwards before one. They do not produce the same list and should not: the boxes on the left of a bowtie are pathways, while root cause analysis produces explanations.